Kinetic Study of the Electrochemical Mineralization of m-Cresol on a Boron-Doped Diamond Anode

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Flox, Cristina and Brillas, Enric and Savall, André and Groenen-Serrano, Karine

Kinetic Study of the Electrochemical Mineralization of m-Cresol on a

Boron-Doped Diamond Anode. (2012) Current Organic Chemistry, vol. 16 (n° 17). pp.

1960-1966(7). ISSN 1385-2728

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Kinetic Study of the Electrochemical Mineralization of m-Cresol on a Boron-Doped

Diamond Anode

Cristina Flox

, Enric Brillas

, André Savall

and Karine Groenen-Serrano

*

a

Laboratori d’Electroquímica dels Materials i del Medi Ambient, Departament de Química Física, Facultat de Química, Universitat de Barcelona, Martí i Franquès 1-11, 08028 Barcelona, Spain

b

Laboratoire de Génie Chimique, CNRS, Université Paul Sabatier, 118 route de Narbonne 31062 Toulouse cedex 02, France

c

Catalonia Institute for Energy Research, IREC, Jardins de les Dones de Negre 1, 08930 Sant Adrià de Besòs, (Barcelona), Spain

Abstract: The kinetics of the electrooxidation of m-cresol in aqueous solution was investigated in a one-compartment flow

electrochemi-cal cell with a boron-doped diamond electrode (BDD). Cyclic voltammograms recorded on BDD revealed that cresol oxidation takes place at a potential very close to the discharge of water. Under potentiostatic conditions, at a working potential lower than water dis-charge, a passive layer was rapidly formed on the electrode surface due to cresol polymerization. The anode fouling was not observed during electrolysis performed with the flow electrochemical cell operating under galvanostatic conditions. In this case, the decay of m-cresol concentration followed a pseudo-first-order kinetics. The abatement of chemical oxygen demand (COD) showed that the kinetics of m-cresol oxidation was limited by mass transfer and that a full mineralization was achieved. A good agreement between predicted and experimental COD and instantaneous current efficiency values was obtained, although some deviations were observed at high current since the experimental data decreased faster than those predicted ones. These deviations can be explained by the occurrence of oxygen evolution which increases the mass transfer coefficient.

Keywords: Electrooxidation, Boron-doped diamond, Cresol, Wastewater treatment. 1. INTRODUCTION

Aromatic compounds are generally toxic and the products hav-ing a quinonic structure are characterized by very strong toxicities. Aromatic structures are usually very stable and as a consequence, aqueous effluents polluted with such compounds should be decon-taminated by effective physico-chemical techniques of oxidation. Cresols are classified by the US-EPA as persistent, priority and toxic chemicals, showing chronic effects at 12 mg L-1 of the quan-titative structure–activity relationship [1]. Unfortunately, the high contamination level that these compounds reach in industrial wastewaters, their seasonal production and the presence of other organic pollutants such as lipids usually render these effluents inap-propriate for direct biological treatment. Alternative powerful oxi-dation technologies are then needed to be developed for achieving total destruction of cresols from wastewaters.

Several papers reported the removal of cresols by advanced oxidation processes (AOPs) [1-4]. The main features of the electro-chemical oxidation of cresols on BDD and PbO2 were presented in

a previous paper [5]. The use of Fenton’s reagent for cresol oxida-tion is effective during the first steps of the process, but it suffers the formation of by-products like oxalic or acetic acids that limits the mineralization because of the formation of stable complexes with iron ions [6].

Electrochemistry is a convenient technique for the treatment of industrial effluents containing organics. Mineralization of organic compounds can be fully achieved under conditions in which the transfer of oxygen atoms is effective. It is generally assumed that

*Address correspondence of this author at the Laboratoire de Génie Chimique, CNRS, Université Paul Sabatier, 118 route de Narbonne 31062 Toulouse cedex 02, France; Tel: + 33 (0)5 61 55 86 77; Fax: + 33 (0) 5 61 55 61 39;

E-mail: serrano@chimie.ups-tlse.fr

hydroxyl radical (•OH) resulting from water discharge by reaction (1) is particularly active to carry out oxygen atoms transfer [7-10]:

H2O  •OH + H+ + e
(1)

The strong activity of this radical in aqueous solution results from the high value of its standard potential E0 = 2.74 V/SHE [11]. Organic molecules are degraded into smaller molecules by a suc-cession of oxygen atom transfers until complete destruction of the carbon skeleton [10]. In these last 20 years, the use of boron-doped diamond (BDD) has known an increasing interest in the electro-chemical community because of its unique properties. BDD pro-duces hydroxyl radicals that are free on its surface and can react massively close to the electrode allowing the complete mineraliza-tion of organic compounds.

Several papers reported the rapid removal of o-, m- and p-cresol from waters by electro-oxidation using a BDD anode [5, 12, 13]. The behaviour of BDD was numerically predicted by mathematical model developed by Mascia et al. [13] and the effect of the pres-ence of chloride ions on the COD removal was studied.

A previous study [5] showed that the electrochemical incinera-tion of m-cresol on a BDD anode occurs by giving by-products in very weak concentration. Furthermore, it was found that the PbO2

anode is comparatively much less efficient due to the lower oxida-tion ability of hydroxyl radicals that it is able to produce under the same operating conditions.

The aim of this work is to gain a better insight into the kinetics of the electrochemical mineralization of m-cresol in a one-compartment flow electrochemical reactor with a BDD anode. The influence of operating parameters such as applied current, initial m-cresol concentration and electrolyte flow rate was examined. The results obtained are presented and discussed in terms of m-cresol decay, current efficiency and chemical oxygen demand (COD)

removal. Cyclic voltammetry was also used to ascertain the nature of the oxidation process as a function of the applied potential.

2. EXPERIMENTAL 2.1. Chemicals

m-Cresol (99% purity) was purchased from Sigma and used as

received. Solutions were prepared with high-purity water obtained from a Millipore Milli-Q system (resistivity > 18 M
cm at 25 ºC). Analytical grade sulfuric acid from Prolabo was used to adjust the initial pH to 4.0. Anhydrous sodium sulfate employed as supporting electrolyte was analytical grade supplied by Prolabo. Other chemi-cals and organic solvents were either HPLC or analytical grade from Sigma-Aldrich, Merck and Prolabo.

2.2. Cyclic Voltammetric Study

Cyclic voltammograms were carried out in a conventional three-electrode cell of 100 mL capacity using a computer controlled Autolab potentiostat Model 30. A BDD thin film of 0.196 cm2 from Adamant was used as working electrode, a Pt wire as counter elec-trode and a Hg/Hg2Cl2,Cl

-(sat) electrode (SME) as the reference electrode. Before each experiment, the electrode was pre-treated using anodic polarisation at 2.84 V/SHE during 40 s into a 1 M H2SO4 solution.

2.3. Electrochemical Reactor

Electrolyses were conducted in a one-compartment flow filter-press reactor under galvanostatic conditions (Fig. 1) The electrolyte was stored in a 1 liter thermoregulated glass reservoir (1) and circu-lated through the electrolytic cell using a centrifugal pump (2). Electrodes were two discs of 63 cm of active surface separated 10 mm. The BDD anode from Adamant was elaborated by chemical vapour deposition on a conductive substrate of silicium. The cath-ode was a 1 mm thick disc of zirconium. The current was supplied by an ELC AL 924 power supply.

2.4. Analysis Procedures

Samples withdrawn from treated solutions were filtered with Whatman 0.45
m PTFE filters before analysis. The mineralization process was monitored from COD decay. COD was determined by photometry by means of test tubes ready for use and of a pocket photometer (Dr Changes Lasa 50 system).

m-Cresol concentration was followed by reversed-phase HPLC

chromatography using a Hewlett-Packard 1100 Series liquid chro-matograph, fitted with a Spherisorb ODS2 5
m, 150 mm x 4.6 mm (i.d.), column at room temperature, equipped with an UV/Vis detec-tor selected at  = 276 nm and controlled with an Agilent ChemSta-tion software. These measurements were made by injecting 20
L aliquots and circulating a 50:45:5 (v/v/v) methanol/phosphate buffer (pH = 2.5)/pentanol mixture at 1.0 mL min-1 as mobile phase.

2.5. Kinetic Regimes

The kinetics of the electrode processes can be controlled either by mass transfer or by charge transfer. For a constant current den-sity higher than the limiting current denden-sity, the process rate is lim-ited by mass transport to the electrode. In that case, oxygen evolu-tion is the main secondary reacevolu-tion occurring on the anode. In con-trast, for a constant current density lower than the limiting value, the global reaction is limited by charge transfer. The initial limiting current density is calculated as follows:

ilim 0 ₌

z F k_dC0

(2) where z is the number of exchanged electrons for the electrode process, F is the Faraday constant (96,487 C mol-1), kd is the mass

transfer coefficient (m s-1) and C0 is the initial concentration of the substrate (mol m-3).

While the z number is not exactly known for an organic com-pound oxidized on a PbO2 anode characterized by a high-oxygen

overvoltage [14,15], in the case of the BDD anode, the z number is known and the concentration C0 in Eq. 2 can be replaced by the initial COD expressed in mol of O2 m-3 [16] as giving in Eq. 3:

ilim 0 ₌

4 F k_dCOD° (3)

Under galvanostatic conditions, two different operating regimes are defined:

o When the applied current (I) is lower than the limiting cur-rent (Ilim), the electrolysis is controlled by the charge transfer,

the current efficiency is 100% and the rate of COD removal is constant and can be written as:

COD t

( )

In Eq. 4,  is defined as the ratio I/Ilim, t is the electrolysis time

and  is a time characteristic of the device defined as  = V/(kd A),

where V the volume of the solution to be treated (m3) and A is the electrode active area (m2).

o If the applied current exceeds the limiting current ( > 1), the process is mass transfer controlled. In this regime, COD de-creases exponentially with time according to Eq. 5.

COD t

( )



 (5)

Secondary reactions such as oxygen evolution take place and the instantaneous current efficiency (ICE) is lower than 100%. This parameter is calculated from COD values from Eq. (6).

ICE=CODexp

CODth

=4 F V COD t

( )

I t M_O

2

(6)

Fig. (1). Discontinuous process with a single compartment electrochemical

reactor, (1): 1 L tank, (2): centrifugal pump and (3): electrochemical cell.

+

-

where COD(t) and COD(t +
t) are the concentrations of COD expressed in g L-1 at times t and t +
t, respectively,

MO2is the

dioxygen molar weight (32 g mol-1) and V is the volume of solution (L).

3. RESULTS AND DISCUSSION 3.1. Cyclic Voltammetric Study

The electrochemical behavior of m-cresol in sodium sulfate so-lution was studied by cyclic voltammetry on the BDD electrode. Fig. (2) shows two successive scans recorded between -0.5 and 1.5 V at 25 °C. During the first scan towards positive potentials, a well defined peak which corresponds to m-cresol oxidation appears be-fore water oxidation (see dashed line). Fig. (2) also shows that elec-trode passivation occurred after the first cycle since no oxidation peak was observed in the second cycle. This behavior is due to the formation of a passive layer of polymers at the surface of the work-ing electrode.

Various cyclic voltammograms were also carried out at differ-ent concdiffer-entrations of m-cresol. In each case, only the first scan gave an oxidation peak at similar potentials to those shown in Fig. (2). After each scan, the electrode activity was restored using anodic polarisation at 2.84 V during 40 s. The variation of the anodic peak current as function of m-cresol concentration is shown in Fig. (3).

Fig. (3) depicts that at concentrations of m-cresol lower than 5 mM, the peak current varies linearly with the concentration in the solution. In contrast, at concentrations higher than 5 mM, the peak current is stabilized due to the formation of a polymer film spread on the electrode surface. This passive film blocks the electrode activity. Indeed, a visual examination of the electrode surface re-vealed the presence of an adherent film of polymer (see micrograph in Fig. 4).

Fig. (4). Optical micrograph of the BDD surface after the cyclic

voltammo-grams of Fig. (2). The black zone corresponds to BDD deposit and the white spots to deposited polymers.

Fig. (2). Consecutive cyclic voltammograms (scans 1 and 2) recorded for 10 mM m-cresol in 0.5 M Na2SO4. The dashed curve corresponds to the electrolyte solution. Working electrode: BDD (0.196 cm
), counter electrode: Pt and reference electrode: MSE. Scan rate: 50 mV s-1

.

Fig. (3). Variation of the anodic peak current for the oxidation peak found during the first cyclic voltammetric scan as a function of m-cresol concentration.

Working electrode: BDD (0.196 cm
), counter electrode: Pt and reference electrode: MSE. Scan rate: 50 mV s-1 . -5.00E-05 5.00E-05 1.50E-04 2.50E-04 -1 -0.5 0 0.5 1 1.5 2

I /

A

E vs SME/V

Scan 1

Scan 2

Na

SO

I /

m

A

[m-cresol] /mM

Similar observations have been noted using BDD anodes in the case of phenol [17], 2-naphtol [18] and 4-chlorophenol [19]. In these papers, it is assumed that the electron transfer to the phenolic ring leads to phenoxy radicals that react to form polymeric struc-tures by coupling reactions. This behavior could also be considered for m-cresol oxidation at potentials lower than those of water dis-charge.

3.2. Electrolysis in the Flow Cell Under Galvanostatic Condi-tions

Fig. (5) shows that the kinetics of the oxidation of 534 ppm of

m-cresol in 0.05 M Na2SO4 using the flow electrochemical cell with

a BDD anode is enhanced when the temperature rises. So, the com-pound was completely removed in 420 min at 25 ºC, but only in about 300 min at 60 ºC operating at 2.5 A and liquid flow rate of 126 L h-1. Under these conditions, the BDD reached potentials in the water discharge region and no polymers were deposited on it.

According to our previous study [5], the above concentration decays were well-fitted to a pseudo-first-order kinetic equation. A quite similar kinetic behavior has also been described by Polcaro et

al. [20] for the removal of phenol solutions with BDD in

gal-vanostatic electrolysis. The inset panel of Fig. (5) confirms the first-order kinetics for the anodic oxidation of m-cresol, allowing

deter-mining the apparent kinetic constant (k) for the above temperatures. The k-value measured for the same ratio electrode surface/volume treated was 1.69
10-4

s-1 for 25 ºC and 3.19
10-4 s-1 for 60 ºC. These data allow estimating an activation energy of 15 kJ mol-1, which is in agreement with a kinetic process limited by mass trans-fer [21, 22].

Fig. (6) highlights the influence of the flow rate on COD decay at the same operating parameters. As can be seen, the COD removal becomes more effective as the flow rate of the electrolyte is higher, indicating that the process is mass transport limited. For the electro-chemical reactor, the following relation between the flow rate of the solution (u, in L h-1) and the mass transfer coefficient (kd, in m s-1)

was determined by means of the ferri/ferro redox system under the same hydrodynamic conditions [16]:

kd ==2.78
10-6 u0.38 (7)

Note that at 25°C, for an electrolysis carried out at I = 2.5 A us-ing a flow rate of 126 L h-1, m-cresol decays in 87% at t = 200 min (Fig. 5) while the related COD removal is smaller, close to 75% (Fig. 6). Comparison of these m-cresol and COD removals confirms the almost direct conversion of the initial pollutant into carbon di-oxide and water using a BDD anode, in agreement with the very few amounts of by-products reported by us previously [5]. Indeed,

Fig. (5). Variation of m-cresol concentration during the electrolysis of 1 L of a 534 ppm m-cresol solution in 0.05 M Na2SO4 using a flow electrochemical cell with a BDD anode at 2.5 A, temperatures of 25 and 60 ºC and liquid flow rate of 126 L h-1

. The inset panel shows the kinetic analysis for the related experi-ments assuming a pseudo-first-order reaction for each temperature.

Fig. (6). COD abatement during the electrolysis of 1 L of a 534 ppm m-cresol solution in 0.05 M Na2SO4 using a flow electrochemical cell with a BDD anode at 2.5 A and 25 °C for electrolyte flow rates of 42, 126 and 252 L h-1

. The inset panel shows the kinetic analysis for the related experiments assuming a pseudo-first-order reaction for each flow rate.

0 200 400 600 0 100 200 300 400 500

[m

-cre

so

l]

/m

g

L

time /min

this study highlighted that the temporal evolution of the experimen-tal dissolved organic carbon (DOC) obtained during galvanostatic electrolysis of a m-cresol solution using BDD was similar to that calculated from the m-cresol concentration.

Consequently, it is convenient to use the global parameter of COD and ICE to monitor the detoxification of the solution during the electrolysis (Eq 4-6).

Fig. (7) shows the decrease of the normalized chemical oxygen demand of m-cresol solutions at different initial concentrations during electrolysis carried out at 2.5 A, 25 ºC and electrolyte flow rate of 126 L h-1. The m-cresol concentrations and the constant current density were chosen in such a way thatthe process is lim-ited by the mass transfer to the electrode from the beginning of the electrolysis (except for m-cresol = 1253 ppm, I = Ilim at 168 min).

Fig. (7) evidences the same variation of the normalized COD with time whatever the initial concentration of m-cresol in the solution, reaching always overall decontamination in about 500 min. Moreo-ver, Fig. (7) shows a very good agreement between the experimen-tal and theoretical (given by Eq. 4-5) COD values. Thus, this model allows predicting the change of COD for any m-cresol solution. The COD variation then follows a first-order kinetics whatever the ini-tial concentration because a m-cresol molecule reaching the BDD anode is almost totally mineralized, without significant formation and diffusion of by-products towards the bulk.

Fig. (8) shows the variation of the m-cresol concentration as a function of time for the same experiments of Fig. (7). The inset panel in Fig. (8) depicts that the concentration decay always verifies a pseudo-first-order kinetics and that the rate of disappearance of

m-cresol becomes faster for smaller initial concentration. The

k-values thus obtained were 2.16
10-4_{, 1.69
10}-4_{, 1.50
10}-4

and 1.33
10-4

s-1 for 246, 534, 790 and 1253 ppm, respectively. Never-theless, at a given time, increasing concentrations yield the removal of more amount of m-cresol.

These results can be explained as follows. The decrease in m-cresol concentration leads to an increasing part of the current used for water discharge by reaction (1). Thus, a higher production of oxygen bubbles occurs close to the electrode by the following sequence: • OH + •_OH 
k H1 2O2 (8) H2O2+ •OH k2 
 H2O+ O2•-+ H+ (9) O2•-+ •OH k3 
 O2+ OH- (10)

The absolute constant rates for these reactions are k1 = 5.2
109

M-1s-1 [23, 24], k2 = 2.7
107 M-1s-1 [25, 26] and k3 = 1010 M-1s-1

[27, 28]. Consequently, the mass transfer coefficient increases lo-cally. Accordingly, the kinetic constant k = kd A/V increases with

decreasing m-cresol concentration.

Fig. (7). Variation of normalized COD calculated (lines) and experimental (symbols) for the electrochemical oxidation of solutions containing m-cresol

con-centrations of 246, 534, 790 and 1253 ppm at 2.5 A, 25 º C and electrolyte flow rate of 126 L h-1 .

Fig. (8). Decay of m-cresol concentration for the experiments reported in Fig. 7.The kinetic analysis for the related experiments considering a pseudo-first-order reaction for each initial concentration is depicted in the inset panel.

0 0.25 0.5 0.75 1 0 200 400 600 246 pm 534 ppm 790 ppm 1253 ppm theoretical values

CO

D /C

O

D

°

time /min

0

400

800

1200

1600

0

250

500

[m

-c

re

sol]

/mg L

time /min

It is important to compare the experimental variation of COD and the theoretical values predicted by theory (Eq. 4-6) for both operating regimes:

• I < Ilim° (1.76 A). For this case, Fig. 9 highlights the

exis-tence of two consecutive different kinetic zones. At the begin-ning of the electrolysis (specific charge (Q) < 0.78 Ah L-1, t < 31 min) and according to electron transfer control, COD varies linearly according to Eq. 4. However, at times longer than tc =

31 min, COD decreases according to an exponential decay, characteristic of a process limited by mass transfer (Eq. 5). Moreover, theoretical and experimental values for both COD and ICE (inset panel of Fig. 9) are in quite good agreement.

• I > Ilim°. Fig. 10 shows the variation of COD during

gal-vanostatic electrolysis at a current widely higher than the initial limiting current. As can be seen, the experimental values are lower than those calculated from the model (Eq. 5, dashed line in Fig. 10). A similar behavior is observed for the instantaneous current efficiency (inset panel of Fig. 10). To explain these de-viations, it can be taken into account that during the electroly-sis, the depletion of organic matter leads to an increase of oxy-gen evolution at the anode, as pointed out above. Consequently, the rate of gas evolution close to the electrode has a great effect on mass transfer since a release of bubbles increases the mass

transfer coefficient [29]. Beck [30] showed that the mass trans-fer coefficient k_d is the sum of two contributions, the flow rate

k_dh

and the convection due to bubbles k_db

, according to Eq. 11:

k_d=k_dh+k_db (11) Nevertheless, Vogt [29] tested and proposed a more precise cor-relation given by Eq. 12, which takes into account the influence of the flow rate on the bubble radius before dropping out:

0.5 2 b d k h d k 1 b d k d k                   
+ = (12) k_db

depends on the gas flow production [29] :

5 , 0 m G b d D S V const k         
= • (13)

In this equation, D is the gas diffusion coefficient in liquid phase, V_G

•

is the volumetric rate of gas evolution and S area of the flow cross section. The constant value m is function of the gas

na-Fig. (9). Variation of COD and ICE (inset panel) with specific current during the electrochemical oxidation of 534 ppm m-cresol in 0.05 M Na2SO4 on a BDD anode. Operating conditions: I = 1.5 A, Ilim°= 1.76 A, tc = 31 min, T = 25 °C and electrolyte flow rate = 126 L h

-1

. Experimental points (symbols) and theoreti-cal variation (solid line).

Fig. (10). Change of COD and ICE (inset panel) with specific current during the electrochemical oxidation of 534 ppm m-cresol in 0.05 M Na2SO4 Na2SO4 on a BDD anode. Operating conditions: I = 5 A, T = 25 °C and electrolyte flow rate = 126 L h-1

. Experimental points (symbols) and theoretical variation (dashed line: kd = k

h

d = 1.75
10 5

m s-1

, solid line: kd corrected = 3.5
10 -5 m s-1 (from Eq. 12)). 0 500 1000 1500 0 5 10 15 20 Q /Ah L-1 CO D/m g L -1 0 25 50 75 100 0 5 10 15 20 ICE /% Q/Ah L-1 0 400 800 1200 1600 0 10 20 30 40 Q /Ah L-1 CO D /mg L -1 0 20 40 60 80 0 6 12 18 24 30 ICE /% Q/ Ah L-1

ture. The average value for dioxygen in acid media is estimated to m=0.5 [30, 31].

The gas flow production, VG

•

S , is a function of the part of the

applied current which produces dioxygen,

iO2, and the molar

vol-ume of dioxygen, Vmol :

V_G • A
      = i_O 2 n F Vmol (14)

The constant of eq. 13 was estimated to 5.4 10-3 with experi-mental data resulted from an electrolysis controlled by mass trans-fer à t=3h.

For Fig. (10), a k_d value of 3.5
10-5 m s-1 was estimated from Eq. 12 to 14, instead of k_dh _{= 1.75
10}-5

m s-1. Taking this corrected kd value, the theoretical COD and ICE decays show an excellent

fitting with the experimental points, as can be seen in Fig. (10).

CONCLUSIONS

The anodic oxidation of m-cresol on BDD at a potential lower than the water discharge promotes the formation of polymeric films quickly fouling the electrode. The electron transfer leads to phe-noxy radicals that react to yield polymeric structures by coupling reactions. The formation of this non conductive polymer is fast and makes that the current of its anodic peak reaches a limiting value for a concentration of 5 mm. The variation of m-cresol concentra-tion during electrolysis performed in galvanostatic mode using a flow electrochemical cell with a BDD anode corresponds to a first-order kinetics, in accordance with a process limited by mass trans-fer. Also, the effect of the electrolyte flow rate on the mass transfer coefficient shows that m-cresol oxidation is limited by diffusion. Experiments carried out at different temperatures allowed estimat-ing an activation energy of 15 kj mol-1, a rather low value which confirms a diffusion-controlled process. The COD variation evi-dences the excellent performance of BDD anode towards the min-eralization of m-cresol. Practically all the flow of organic matter arriving at the electrode is instantaneously mineralized. The mass transfer is widely increased by oxygen evolution when the elec-trolysis is carried out at current higher than its limiting value.

CONFLICT OF INTEREST

The author(s) confirm that this article content has no conflicts of interest.

ACKNOWLEDGEMENT

Declared none.

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